Heretofore, as a technique for obtaining a specific gas from a mixed gas, there is known a separation method by the use of an organic or an inorganic gas separating film. Among the separating films, examples of a hydrogen separating film for use in a film separation method include organic polymeric films of polyimide, polysulfone and the like, and inorganic compound films of palladium, palladium alloys and the like, and examples of an oxygen separating film include films of silver and silver alloys. The palladium film and the palladium alloy films have heat resistance and can obtain extremely high-purity hydrogen.
Palladium and the palladium alloys have characteristics which allow hydrogen to be dissolved therein and which allow hydrogen to permeate therethrough, and by the utilization of the characteristics, a thin film comprising palladium or the palladium alloy has been widely used as a gas separator for separating hydrogen from a mixed gas containing hydrogen. However, the thin film comprising palladium itself is weak in mechanical strength, and so, in Japanese Patent Application Laid-open No. 273030/1987, palladium or the palladium alloy is deposited on the surface of an inorganic porous support of a porous glass, porous ceramics, a porous aluminum oxide or the like to increase the mechanical strength of the thin film comprising palladium or the palladium alloy.
Japanese Patent Application Laid-open No. 146122/1991 discloses a method for preparing a hydrogen separator which comprises first forming a palladium thin film on the surface of a heat-resistant porous substrate by a chemical plating process, and further forming a silver thin film on the palladium thin film by the chemical plating process, followed by a heat treatment. According to this disclosed method, the hydrogen separator having the porous substrate and the palladium alloy thin film covering it can be obtained. In this palladium alloy thin film, palladium and silver are uniformly distributed by the above-mentioned heat treatment.
In addition, U.S. Pat. No. 3,359,705 discloses a silver thin film for separating oxygen.
However, these gas separators have a drawback that a material gas to be subjected to the gas separation leaks into a purified gas through holes (hereinafter referred to as "throughhole-defects") which extend through the gas separating film comprising the metal for separating the gas. Therefore, the concentration of hydrogen in the purified gas deteriorates as much as the leaked material gas. For example, Japanese Patent Application Laid-open No. 273030/1987 discloses a method for preparing a hydrogen separating film using palladium or the palladium alloy in which the surface of an inorganic porous material is chemically activated, and the palladium thin film is then deposited thereon by the chemical plating process. However, the palladium film formed by the chemical plating process has the holes extending through the palladium film, and the material gas inconveniently flows into the purified gas through these holes. In the method disclosed in this publication, the palladium film is formed on the surface of the porous material, but this palladium film is not formed in the pores.
Furthermore, in Japanese Patent Application Laid-open No. 171617/1988, there is disclosed a method for preparing a hydrogen separating film in which palladium is supported on an inorganic porous film, and this method comprises vapor-depositing palladium or the palladium alloy on the inorganic porous film by sputtering or the like, immersing it in an aqueous [Pd(NH.sub.3).sub.4 ]Cl.sub.2 solution, and then carrying out a vacuum deaeration treatment via the inorganic porous film to vaporize a solvent, thereby supporting palladium on the inorganic porous film. However, according to an example, this hydrogen separating film allows not only hydrogen but also nitrogen to permeate therethrough, and therefore it is apparent that the pores in the inorganic porous film are not closed with palladium. Moreover, palladium is held on the porous film by depositing it or vaporizing the solvent from the aqueous palladium solution, but the laid-opened publication has not any description regarding a chemical plating method for reducing divalent palladium ions contained in [Pd(NH.sub.3).sub.4 ]Cl.sub.2.
The silver thin film disclosed in U.S. Pat. No. 3,359,705 and the hydrogen separator described in Japanese Patent Application Laid-open No. 146122/1991 have a similar problem that the material gas flows into the purified gas.
In order to remove these throughhole-defects, there is a method of thickening the gas separating film comprising the metal for separating the gas, but this method has a problem that a gas permeability of the gas separating film deteriorates and hence a gas separation efficiency also deteriorates.
Additionally, this method also has a problem that adhesive properties between the gas separating film and a substrate such as the porous film are weak, and when the hydrogen separator obtained by the method is actually used in a gas separation process, the gas separating film peels in a short period of time. In consequence, such a hydrogen separator cannot be used continuously for a long term in order to do the gas separation.